1. Field of the Invention
The present invention relates to a welded can of tin-free steel having a coated seam, the adhesion of a protecting resin coating to the welded seam being excellent and the anti-corrosive action of the coated seam being maintained for a long time even after processing such as flanging, double seaming, beading or necking.
2. Description of the Prior Art
As the conventional process most popularly adopted for the manufacture of can bodies, there can be mentioned a process comprising forming a metallic can blank cut in a predetermined size into a cylinder, lapping both the edges of the blank and bonding the lapped edges to form a lap seam, a lock seam or a combination thereof by soldering or using an adhesive.
According to this can forming process, a considerable area is necessary for the seam portion, and the process therefore includes a problem in connection with conserving the material used. Furthermore, in case of soldered cans, bad influences are brought about by soldering. In case of bonded cans, the strength of the seam and the durability thereof are inadequate. Moreover, in seamed cans prepared by soldering or using an adhesive, since there is inevitably formed a considerable step in the side seam portion, leakage is often caused in this step portion when such can bodies are double-seamed with lids.
Instead of soldered cans, seamless cans prepared by draw-ironing have been used in certain fields. However, deformation of the side wall by application of pressure is extreme in seamless cans, and hence, they cannot be conveniently used as vacuum cans, that is, cans of the type where the canned contents are subjected to retort sterilization.
As another instance that can be used instead of the soldered can, there is known a welded can formed by lapbonding both the side edges of a can body by welding. The area of the lap seam portion in such welded can is much smaller than in the soldered can since the thickness of the seam is relatively small, and leakage from the step portion is moderated. Moreover, this lap bonding method is advantageous in that a particular adhesive, for example, a solder, need not be used. However, the preparation operations are very troublesome when certain can materials are used. Furthermore, known welded seam cans are still inadequate in corrosion resistance of the seam, coating adhesion and appearance of the seam.
For example, tin-free steel obtained by subjecting a rolled steel plate to an electrolytic treatment with chromic acid is cheaper and available more easily than other can blanks such as tinplate, and a problem of dissolution of tin into the contents does not arise in cans of tin-free steel. Moreover, tin-free steel is advantageous in that it has excellent corrosion resistance and coating adhesion. In order to form a welded seam on this tin-free steel (often referred to as "TFS" hereinafter), however, it is necessary to remove a chromium-containing layer having a high electric resistance before welding, and therefore, a thick layer of a porous steel oxide is inevitably formed on the exposed steel surface formed by removal of the chromium-containing coating layer and steel molten by the pressure applied at the welding step extrudes into the sectional portion of the seam and such extruding undergoes oxidation more readily.
This steel oxide is brittle and porous. Accordingly, if a protecting resin coating is applied to this steel oxide, adhesion is poor and dissolution of iron into the contents or corrosion of the seam is hardly prevented. Such defects of TFS welded cans, that is, poor adhesion to the coating and poor corrosion resistance, are made more serious when welded seams are subjected to processing necessary for the manufacture of cans, such as flanging, double seaming, necking or beading or when cans are subjected to a severe hydrothermal treatment such as sterilization after packing of the contents.
This steel oxide layer causing the foregoing disadvantages can be removed by pickling of TFS cans after welding. However, the running and installation costs are drastically increased by subjecting respective welded cans to pickling, but in the welded cans after the pickling treatment, it has been found that the adhesion of the protecting resin coating to the seam is not improved as expected and the anti-corrosive effect on seams, especially the anticorrosive effect after processing, is not significantly improved.